The present invention relates to a process for the gas-phase polymerization of olefins carried out in two interconnected polymerization zones, to which one or more xcex1-olefins CH2xe2x95x90CHR are fed in the presence of a catalyst under polymerization conditions and from which the produced polymer is discharged. In the process of the present invention the growing polymer particles flow through a first polymerization zone under fast fluidization conditions, leave said first zone and enter a second polymerization zone through which they flow in a densified form under the action of gravity, leave said second zone and are reintroduced into the first polymerization zone, thus establishing a circulation of polymer between the two polymerization zones.
The development of catalysts with high activity and selectivity of the Ziegler-Natta type and, in more and more applications, of the metallocene type, has led to the widespread use on an industrial scale of processes, in which the polymerization of the olefins is carried out in a gaseous medium in the presence of a solid catalyst. Compared with the more conventional technology in liquid suspension (of monomer or of monomer/solvent mixtures), this technology has the following advantages:
a) operational flexibility: the reaction parameters can be optimized on the basis of the characteristics of the catalyst and of the product and are not limited by the physico-chemical properties of the liquid mixtures of the reaction components (generally including hydrogen as a chain transfer agent);
b) widening of the product range: the effects of swelling of the growing polymer particle and of solubilization of polymer fractions in a liquid medium greatly reduce the range of production of all the kinds of copolymers;
c) minimization of the operations downstream of the polymerization: the polymer is obtained directly from the reactor in the form of dry solid and requires simple operations for removing dissolved monomers and deactivating the catalyst.
All the technologies devised hitherto for the gas-phase polymerization of xcex1-olefins provide for maintaining a bed of polymer, through which the reaction gases flow; this bed is maintained in suspension either by mechanical stirring (stirred-bed reactor) or by fluidization obtained by recycling the reaction gases themselves (fluidized-bed reactor). In both the reactor types, the monomer composition around the polymer particle in the reaction is maintained sufficiently constant owing to the induced stirring. Said reactors approximate very closely the ideal behaviour of the xe2x80x9ccontinuous stirred-tank reactorxe2x80x9d (CSTR), making it relatively easy to control the reaction and thereby ensuring consistency of quality of the product when operating under steady-state conditions. What is by now the most widely established industrial technology is that of the fluidized reactor operating under xe2x80x9cbubblingxe2x80x9d conditions. The polymer is confined in a vertical cylindrical zone. The reaction gases exiting the reactor are taken up by a centrifugal compressor, cooled and sent back, together with make-up monomers and appropriate quantities of hydrogen, to the bottom of the bed through a distributor. Entrainment of solid in the gas is limited by an appropriate dimensioning of the upper part of the reactor (freeboard, i.e. the space between the bed surface and the gas offtake), where the gas velocity is reduced, and, in some designs, by the interposition of cyclones in the exit gas line.
The flow rate of the circulating gas is set so as to assure a fluidization velocity within an adequate range above the minimum fluidization velocity and below the xe2x80x9ctransport velocityxe2x80x9d. The heat of reaction is removed exclusively by cooling the circulating gas. The catalyst components are fed in continuously. The composition of the gas-phase controls the composition of the polymer. The reactor is operated at constant pressure, normally in the range 1-3 MPa. The reaction kinetics are controlled by the addition of inert gases.
A significant contribution to the reliability of the fluidized-bed reactor technology in the polymerization of xcex1-olefins was made by the introduction of suitably pretreated spheroidal catalyst of controlled dimensions and by the use of propane as diluent (see WO 92/21706). Fluidized-bed technology has limits, some of which are discussed in detail below.
A) Removal of the Heat of Reaction
The maximum fluidization velocity is subject to quite narrow limits (which already entail reactor volumes for disengagement which are equal to or greater than those filled by the fluidized bed). Depending on the heat of the reaction, the polymer dimensions and the gas density, a limit to the productivity of the reactor (expressed as hourly output per unit reactor cross-section) is inevitably reached, where operation with gas inlet temperatures higher than the dew point of the mixture of the gases is desired. This limit can lead to reductions in the plant output, in particular in the copolymerization of ethylene with higher xcex1-olefins (hexene, octene), which is carried out with conventional Ziegler-Natta catalysts, requiring gas compositions rich in such olefins. Many ways of overcoming the limits, in terms of heat removal, of the traditional technology have been proposed, based on partial condensation of the circulating gases and on the use of the latent heat of evaporation of the condensates for controlling the temperature in the interior of the reactor (see EP-89691, U.S. Pat. No. 5,352,749, WO 94/28032). Although technically worthy of consideration, all the systems proposed for implementing the principle render the operation of the fluidized reactors critical.
In particular (and apart from problems associated with the distribution of wet solids in the plenum below the distribution grid), the technology used in patents EP-89691 and U.S. Pat. No. 5,352,749 relies on the turbulence generated by the grid to distribute the liquid over the polymer. Possible coalescence phenomena in the plenum can give rise to uncontrollable phenomena of poor distribution of liquid with formation of agglomerates which can not be redispersed, in particular in the case of polymers which have a tendency to stick. The discrimination criterion given in U.S. Pat. No. 5,352,749 reflects situations under steady-state conditions, but offers no feasible guide for situations of even a transient xe2x80x9creaction runawayxe2x80x9d, which can lead to irreversible loss of fluidization, with a consequent collapse of the reactor.
The method described in patent WO 94/28032 involves separation of the condensates and their distribution above the grid by means of special, suitably located nozzles. In fact, the condensates inevitably contain solids in reactive conditions, whose concentration can become very high at low condensate amounts. Moreover, the inherent difficulty of uniformly distributing a suspension over a number of nozzles can compromise the operability of some of them and a blocking in one nozzle adversely affects the distribution of the liquid evaporating in the relevant section of the reactor. It is also clear that the efficiency of the operation depends upon a vigorous circulation of solids in the reactor and, below the injection points, this is reduced by an unbalancing of the gas flow rates caused by large quantities of condensates. Furthermore, any need for maintenance on one nozzle requires a complete shut-down of the reactor.
B) Molecular Weight Distribution
As already stated, a fluidized bed shows a behaviour directly comparable with an ideally mixed reactor (CSTR). It is generally known that, in the continuous polymerization of xcex1-olefins in a single stirred stage (which also involves steady composition of the monomers and of the chain transfer agent, normally hydrogen) with Ti catalysts of the Ziegler-Natta type, polyolefins having a relatively narrow molecular weight distribution are obtained. This characteristic is even more emphasized when metallocene catalysts are used. The breadth of the molecular weight distribution has an influence both on the rheological behaviour of the polymer (and hence the processability of the melt) and on the final mechanical properties of the product, and is a property which is particularly important for the (co)polymers of ethylene.
For the purpose of broadening the molecular weight distribution, processes based on several reactors in series, in each of which it becomes possible to operate at least at different hydrogen concentrations, have gained industrial importance. A problem typically encountered also with these processes, when a very broad molecular weight distribution is required, is an insufficient homogeneity of the product. Particularly critical is the homogeneity of the material in blow-moulding processes and in the production of thin films, in which the presence of even small quantities of inhomogeneous material brings about the presence of unfused particles in the film (xe2x80x9cfish eyesxe2x80x9d). In patent application EP-574,821, a system of two reactors is proposed which operate at different polymerization conditions with mutual recirculation of polymer between the two. Even if the concept is suitable for solving the problem of the homogeneity of the product, as shown by the experimental results, such a system involves investment costs and a certain operational complexity.
In other cases, polymers of broad molecular weight distribution are obtained by the use of mixtures of different Ziegler-Natta catalysts in a single reactor, each catalyst being prepared so as to give a different response to hydrogen. It is clear that a mixture of granules each with its own individuality are obtained at the exit from the reactor. It is difficult to obtain homogeneity of the product by this route.
C) Discharge of the Product
The technology of polymerizing xcex1-olefins in gas-phase reactors has rapidly developed in the last years, and the range of polymers obtainable in this way has widened greatly. In particular, besides homopolymers of ethylene and propylene, a wide range of copolymers can be produced industrially, for example:
random copolymers of propylene/ethylene, propylene/ethylene/higher xcex1-olefins and propylene/higher xcex1-olefins;
polyethylenes of low and very low density (LLDPE, VLDPE), modified with higher xcex1-olefins containing 4 to 8 carbon atoms;
heterophasic copolymers of high impact strength, obtained by growth on the active centres of the catalyst, in successive stages, of one or more of the polymers listed above and of ethylene/propylene or ethylene/butene rubbers; and
EPR and EPDM rubbers.
In short, in the polymers producible in the gas phase, the modulus of flexibility varies from 2300 MPa to values lower than 100 MPa, and the xylene-soluble fraction varies from 1% to 80%. The flowability, compactability and sticking properties turn out to be extremely variable as a function of the degree of crystallinity, of the molecular weight and of the composition of the various polymer phases. Many of these products remain granular and flowable (and hence processible) as long as they are maintained in a fluidized state or in flux, which are conditions under which the static forces between the individual solid particles have no effect. They tend more or less rapidly to clump together and to form aggregates if they are allowed to settle or to be compacted in stagnant zones; this phenomenon is particularly enhanced under reaction conditions where, due to the combined action of the temperature and the large quantity of dissolved hydrocarbons, the polymer is particularly soft, compressible and compactable, and sticky. The characterization of soft and sticky polymers is efficaciously described in EP-348,907 or U.S. Pat. No. 4,958,006.
The most direct solution for the discharge of the polymer from the reactor consists of a direct discharge from the fluidized bed through a controlled valve. This type of discharge combines simplicity with the advantage of not producing stagnant zones. Where a sufficiently low pressure (in the range 0.5-3 bar gauge) is maintained downstream of the discharge valve, the reaction is virtually stopped either by the temperature reduction due to the evaporation of the monomers dissolved in the polymer or due to the low partial pressure of the monomers in the gas: in this way, any risk in the receiver equipment downstream of the reactor is avoided.
Nevertheless, it is known that the amount of gas discharged with the polymer from a fluidized bed through an orifice reaches very high values as a function of the reactor pressure, of the fluidization velocity, of the density of the solids in the bed, etc. (see, for example: Massimilla, xe2x80x9cFlow properties of the fluidized dense phasexe2x80x9d, in xe2x80x9cFluidizationxe2x80x9d, p. 651-676, eds. Davidson and Harrison, Academic, New York, 1971). High amounts of gas discharged with the polymer represent both investment costs and operating costs, it being necessary to recompress this gas in order to get back to the reactor pressure from the receiver pressure. In many industrial applications, discontinuous discharge systems have thus been installed, with interposition of at least two hoppers in alternating operation. For example, U.S. Pat. No. 4,621,952 describes a discharge system in which the polymer is transferred intermittently and at high differential pressures from the reactor to a settling tank. The momentum of the polymer which, during the filling phase, impinges first on the walls of the settling tank and then on the bed of polymer compacts the material which loses its flowability properties. During the filling phase the pressure in the settling tank rises rapidly to the value of the reactor pressure and the temperature does not change significantly. The reaction proceeds adiabatically at high kinetics. With soft and sticky products, this easily leads to the formation of agglomerates which cannot be granulated, with consequent difficulties with the discharge to the receiving tank below. Analogous observations apply to U.S. Pat. No. 4,703,094.
The limits of the intermittent system are clearly revealed by the proposal for complicated continuous systems. Japanese patent JP-A-58 032,634 provides for the installation of an internal screw in the reactor for compacting the polymer towards the discharge; U.S. Pat. No. 4,958,006 proposes the installation of an extruder, the screws of which are fed directly in the interior of the fluidized-bed reactor. Apart from the complication and the difficulty of industrial application, the systems proposed are in any case altogether inadequate for feeding the polymer to a subsequent reaction stage.
A novel polymerization process has now been found, and this represents a first aspect of the present invention, which allows olefins to be polymerized in the gas phase with high hourly output per unit reactor volume without incurring the problems of the fluidized-bed technologies of the known state of the art. A second aspect of the present invention relates to an apparatus for carrying out this process.
The gas-phase polymerization process of the present invention is carried out in a first and in a second interconnected polymerization zone to which one or more xcex1-olefins CH2xe2x95x90CHR, where R is hydrogen or a hydrocarbon radical having 1-12 carbon atoms, are fed in the presence of catalyst under reaction conditions and from which the polymer produced is discharged. The process is characterized in that the growing polymer particles flow through the first of said polymerization zones under fast fluidization conditions, leave said first polymerization zone and enter the second of said polymerization zones through which they flow in a densified form under the action of gravity, leave said second polymerization zone and are reintroduced into said first polymerization zone, thus establishing a circulation of polymer between the two polymerization zones.
As is known, the state of fast fluidization is obtained when the velocity of the fluidizing gas is higher than the transport velocity, and it is characterized in that the pressure gradient along the direction of transport is a monotonic function of the quantity of injected solid, for equal flow rate and density of the fluidizing gas. Contrary to the present invention, in the fluidized-bed technology of the known state of the art, the fluidizing-gas velocity is maintained well below the transport velocity, in order to avoid phenomena of solids entrainment and particle carryover. The terms transport velocity and fast fluidization state are well known in the art; for a definition thereof, see, for example, xe2x80x9cD. Geldart, Gas Fluidization Technology, page 155 et seqq., J.Wiley and Sons Ltd., 1986xe2x80x9d.
In the second polymerization zone, where the polymer flows in a densified form under the action of gravity, high values of density of the solid are reached (density of the solid=kg of polymer per m3 of reactor occupied by polymer), which approach the bulk density of the polymer; a positive gain in pressure can thus be obtained along the direction of flow, so that it becomes possible to reintroduce the polymer into the first reaction zone without the help of special mechanical means. In this way, a xe2x80x9cloopxe2x80x9d circulation is set up, which is defined by the balance of pressures between the two polymerization zones and by the head loss introduced into the system.